A conventional canister of an evaporated fuel processing system is disclosed in United States patent Application Publication No. 2002/0078931. The canister houses a plurality of adsorbing chambers connected in series. The first adsorbing chamber is connected to a fuel tank and to an intake pipe of an internal combustion engine and the last or outlet adsorbing chamber comprises an atmospheric port.
The fuel vapours, after entering into the canister from the fuel tank, are captured by the adsorbent included in the chambers. As a rule, activated carbon is used as the chamber-filling agent. After the start of the engine the vapours are inducted by negative pressure of the intake pipe and then burnt inside the combustion chamber. Once adsorptive capability of the first chamber is exhausted, the vapours pass through to successive adsorbing chambers, and after exceeding adsorptive capabilities of the outlet chamber the vapours are discharged into the atmosphere.
To ensure that the container meets requirements of related governmental emission standards, e.g the LEV II (Low-Emission Vehicle Program) or the Zero Evap standard, that are used in use in the USA, emission of the evaporated fuel into the atmosphere must be as low as possible. For this purpose the height of adsorbing chambers must be relatively large, as compared with diameters thereof. In addition, the outlet adsorbing chambers is frequently divided by a partition, thus defining the outer and the inner adsorbing layer. Such a partition produces an additional restraint against vapour flow causing redistribution thereof within the entire volume of the adsorbent below the partition.
Unfortunately, the canisters of the above construction may fail meeting the second requirement that is demanded, which is adsorption of fuel vapour that arises while tank refuelling (onboard refueling vapor recovery —ORVR), where the low resistance to vapour flow is required due to much higher rates of evaporated fuel flow. At high intensity of flow the activated carbon in the adsorbing outlet chamber generates high flow resistance, which may lead to unfavourable behaviour of the canister during tank refuelling. In an extreme case, resistance of the canister may even make the refuelling impossible, as the fuel vapour gauge, installed in a pump gun, cuts the fuel flow out. Resistance to fuel vapour flow is even higher for canisters with a partition installed inside the adsorbing outlet chamber.